• From June 1980 to September 1984, forty-five newborns (weight, ≥2000 g), initially presumed normal, were seen with bilious vomiting in the first 72 hours and were prospectively followed up. Nine (20%) required surgical intervention, five (11%) had nonsurgical obstruction such as meconium plug or left microcolon, and the remaining 31 (69%) had idiopathic bilious vomiting. Infants with idiopathic bilious vomiting had a benign transient course and resumed feedings by 1 week of age; 30 of the 31 had normal or nonspecific findings on initial plain abdominal roentgenogram. Specific findings on the initial plain abdominal roentgenogram were noted in five infants, and four (80%) of these had a lesion requiring surgical intervention; 56% (5/9) of neonates with surgical lesions had normal or nonspecific findings on the plain abdominal roentgenograms. None developed bowel ischemia or midgut infarction secondary to a volvulus as they were identified by contrast studies shortly after the initial episode of bilious vomiting. Although the majority of "normal" neonates with bilious vomiting do not have a surgical lesion, this study indicates that 56% of surgical cases will be missed if contrast studies are not done.